A wide variety of devices may be severely damaged if an operation begins to generate heat which is not detected until damage has occurred. Therefore, equipment temperatures are preferably monitored in a wide variety of processes and applications. An increase in temperature which is sensed may be indicative of a bearing failure, a breakdown of the lubricant system or may merely mean that equipment should be cooled so that it continues to operate at a given temperature range. Systems have previously been available in which multiple sensors are all wired together to a central location where it is possible to monitor the temperature of each station. Such systems are, however, quite expensive since either an operator or computer must be used to evaluate the information obtained from the sensors. This situation is exacerbated when the devices being monitored are in motion relative to the location of the display or control equipment such as the journal bearings of railroad cars. Also, some automatic systems which employ a computer may cause an automatic shut down if one sensor sends a high temperature reading whereas in some situations that may not be desirable. In the case of a railroad, sudden stopping of an entire train because of a hot journal box may cause severe damage to the train and the cargo.
Some of the prior art devices have indicated high temperature by merely providing a visual indicator before a preselected temperature is reached. In some forms a spring is released when a fusible alloy reaches its melting temperature. An internal member then protrudes from a bolt head. Such methods are, however, unreliable because bolt heads are typically exposed to a wide variety of soil and contaminants. In some cases, they may prevent exposure of the indicator member completely. In other cases, they may prevent accurate calibration.